Blinking of the eyelids causes the corneas of the eyes to be covered with tear fluid, including a superficial lipid layer, in the form of a thin film. The ability to maintain a stable tear film imparts the ocular surface with qualities relating to clear vision, comfort and defense against injury and infection. Over the time between blinks, the tear film gradually thins out to a point where the tear film loses cohesion and breaks up. The time required for the tear film to break up (hereinafter referred to as BUT (break up time)) can be understood as a function of the dispersion of the lipid layer. The lipid layer functions to protect moisture from evaporating, to help the tear film to be thin and stable, to help lubricate the lid movement across the eye ball, and to prevent the condition known as dry eye.
Dry eye is the hallmark of an unstable tear film condition. Dry eye can be expressed as a result of lipid tear deficiency (LTD) in a subject possessing meibomian gland disease (MGD), or an aqueous tear deficiency (ATD) in a subject possessing lacrimal gland disease. With regard to ATD, the diagnostic criteria rely on the measurement of tear production. Diagnosis of LTD has been indirect, using either meibography to examine the changes in morphology of the meibomian gland, or digital compression to determine meibum expressibility, or meibometry to determine lid margin meibum, or rapid tear evaporation, or dye staining techniques.
Unstable tear film conditions typically are measured as a function of BUT. One of the preferred BUT measuring methods of the prior art is the incorporation of sodium fluorescein to the tear film. The sodium fluorescein gives a yellowish-green color to the tear film and causes the term film to fluoresce in the presence of a portion of the visible light spectrum (typically blue light). Observation of the tear film is made directly by using a slit lamp, having a blue filter to allow for the passage of light having wavelengths of approximately 330 to 400 nm. The elapsed time between a blink and the development of darkened regions in the fluorescent tear film is defined as the BUT.
Another method used to measure BUT uses an observed grid pattern imaged onto the tear film. The observed image quality depends upon the tear film's stability, yielding a smooth surface and concealing the irregularities of the surface of the cornea.
Yet another method of investigating the tear film is through tear interference. Typically, a single static image is obtained to partially quantify the thickness of the tear film, particularly the lipid film. One such static image providing device is the DR-1®, manufactured by Kowa (Tokyo, Japan). Previously, it has been assumed that such static images are roughly representative of, and provide data approximately relating to, lipid film thickness. However, because a static image is taken without being consistently timed from the onset of the blink, it is impossible to determine important information relevant to diagnosing and treating LTD, such as speed and direction of lipid spread or dispersion. Although there are examples in the prior art of continuous video recording of tear interference during blinking, these video images fail to analyze the kinetics of the tear film to indicate tear stability conditions. Moreover, the prior art video imaging fails to quantify color information of the tear interference images, to further indicate tear stability conditions.
Accordingly, a need exists for a method and apparatus for the kinetic analysis of tear interference, particularly for illustrating a tear film dispersion pattern, indicative of a tear stability condition. Additionally, there is a need for a method and apparatus for the kinetic analysis of lipid film spread that illustrates a lipid film dispersion pattern indicative of a tear stability condition. Moreover, there is a further need for the dispersion pattern to provide quantifiable color information.